How will these 150 LEDs be arranged?
The reason I ask is that you will need to think about how you intend to generate characters on a grid of LEDs. I hate to burst your bubble, but I don't think that counters, registers and flip-flops are going to be much help in this application.

Hi Everyone,
i noticed that Noble have asked the same question i intended to. But the twister is that, is there any way i could save the information that is to be displayed in a flash drive so that instead of bringing down the billboard to edit, i can just do that on the flash and plug it back to run. Pls i need Ideas.

It sounds like you want your homework done for you. I apologise if I am wrong but it isn't very convincing when you just straight-up ask for a circuit diagram without putting forward any thoughts/ideas of your own.

First, you should decide upon how you are going to represent your characters.

The least number of "dots" to represent the letters S, O, and T would be a 3x5 grid, like this:
ooo
ooo
ooo
ooo
ooo
Examples:ooooooooooooooo

ooooooooooooooo

oooooooooooooooooo

Typically, an alphanumeric display would represent characters in a 5x7 grid, but your stated requirements make that unnecessary.

So, you will need 15 LED's to display a single character.
For readability's sake, you will need to leave a blank column between characters; the blank row is another five.

Instead of using 50 red, 50 yellow, and 50 green LED's, consider using 50 red/green/yellow LED's, as this will simplify your wiring and mounting considerably, along with making interesting effects possible.

LED driver IC's: consider using ULN2803's. These are reasonably priced, TTL compatible, compact, and contain 8 Darlington pair NPN transistors. ULN2003's are similar, but contain 7 Darlingtons instead. ULN2x04's are CMOS compatible, but so are the ULN2x03's if you add a 6.8k resistor to each control input, whereas the ULN2x04's can't be (practically) used with TTL. You'll probably want to re-use these drivers for another project later. Keep flexibility in mind.